Magnetorheological (MR) fluids are substances that exhibit an ability to change their flow characteristics by several orders of magnitude and in times on the order of milliseconds under the influence of an applied magnetic field. The utility of these materials is that suitably configured electromechanical actuators that use a MR fluid can act as a rapidly responding active interface between computer-based sensing or controls and a desired mechanical output. With respect to automotive applications, such materials are seen as a useful working media in shock absorbers, for controllable suspension systems, vibration dampers in controllable power train and engine mounts, and in numerous electronically controlled force/torque transfer (clutch) devices.
MR fluids are non-colloidal suspensions of finely divided (typically one to 100 micron diameter) low coercivity, magnetizable solids such as iron, nickel, cobalt, and their magnetic alloys dispersed in a base carrier liquid such as a mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquid. MR fluids have an acceptably low viscosity in the absence of a magnetic field but display large increases in their dynamic yield stress when they are subjected to a magnetic field of, e.g., about one Tesla. The iron particles are kept suspended in the liquid by the action of a thixotrope or anti-settling agent. Special additives are also used to reduce oxidation of the base fluid and iron particles, reduce friction, reduce wear, and improve durability.
In the context of automotive applications, MR fluids have been developed to pass shock absorber durability testing, while minimizing settling and in-use thickening. This has largely been accomplished by careful specification of components of the formulation. For example, prior art fluids have used particular types of magnetizable particles and/or particular types of thickening agents to provide consistent properties and to minimize settling of the MR fluid over its life. In addition, typical prior art MR fluids contain additives, such as organomolybdenums, zinc dialkyl dithiophosphate (ZDDP), thiocarbamates, and phosphorous-containing compounds in low concentration (about 1-2%) to minimize in-use thickening and reduce wear of mechanical components.
First generation (Gen1) MR fluids generally have an operating temperature range of −40-70° C., with excursions up to 105° C. The second generation (Gen2) and third generation (Gen3) MR fluids require a wider operating temperature range, specifically −40° C.-130° C. continuous exposure, with up to 150° C. excursions. The Gen2 MR fluids require 25% higher on-state forces, and the Gen3 MR fluids require 100% higher on-state forces. Both the Gen2 and Gen3 MR fluids require a 25% decrease in off-state forces. These requirements must be met without any compromise in durability and settling performance.
While the Gen1, Gen2 and Gen3 fluids pass the standard durability test, the fluids do exhibit varying degrees of thickening at the end of the test. This is evidenced by the fact that at the end of the test, current MR fluids exhibit increases in off-state damping force (measured at 0.25 m/s piston velocity) of around 10% (Gen1), 15% (Gen3) and 30% (Gen2), on average. While these force increases are within the prescribed limit of 50% (at 0.25 m/s piston velocity), it is evident that the performance of these fluids may deteriorate if the test were to proceed beyond the standard limits. As performance demands increase, it is likely that our MR fluids will be required to pass an extended durability test in the near future. Therefore, it is very important to develop new MR fluids that exhibit significantly extended durability performance.
The additives in the MR fluid significantly influence the durability performance of the fluid in shock absorbers, although the precise mechanism by which these additives protect the fluid is not well understood. Consequently, additives that are currently being used in MR fluids have been developed by a trial-and-error method, and the first additive package that provides adequate performance in the durability test has been used without further refinement or optimization. This is primarily due to the fact that durability testing is expensive and time- and resource-intensive.
The present inventors have conducted extensive analytical tests on unused and post-durability MR fluids. The results of these analyses indicate that the durability performance of Gen3 MR fluids is strongly correlated to the level of the antioxidant additive in the MR fluid. The results further indicate that once the level of antioxidant decreases below a critical level, the fluid exhibits thickening, which manifests itself as an increase in off-state damping force.
There is thus a need to develop a MR fluid formulation that has extended durability, and more specifically, there is a need to develop a combination of additives for an MR fluid that prevents fluid thickening for extended durability testing.